Piezoelectric damping means for mechanical vibrations



June 15, 1948.

w. P. MASON 2,443,471 PIEZOELECTRIC DAMPING MEANS FOR MECHANICAL VIBRATIONS Filed March 29, 1945 F/G. Z W

INVENTOR By W P MASON ATTORNEY Patented June 15, 1948 PIEZOELECTRIC DAMPIN G MEANS FOR MECHANICAL VIBRATION S Warren P. Mason, West Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated,

New York,

N. Y., a corporation of New York Application March 29, 1945, Serial N 0. 585,503 29 Claims. Cl. 178-44) This invention relates to mechanical vibratory elements and more particularly to means for damping an undesired mode of vibration in such an element.

The object of the invention is to damp an undesired mode of vibration in a mechanical vibratory element without appreciably afiecting the desired mode.

A mechanical vibratory element such, for example, as is used in mechanical wave filters is It is, therefore, advantageous to eliminate the undesired mode.

In accordance with the present invention this undesired mode is damped by associating a. pair of piezoelectric crystal plates with opposite sides of the mechanical vibratory element and connecting an electrical impedance between the plates. The plates are so poled that the voltages generated therein are additive for the undesired piezoelectric crysare attached to the sides of the bar the other sides of the bar.

'The nature of the invention will be more fully understood from the following detaile description and by reference to the accompanying drawings, in which like reference characters refer to similar or corresponding parts and in which Fig. 1 is a perspective view of an electromechanical wave filter of the cross bar type including damping means in accordance with the invention; and

Fig. 2 is a cross-sectional view of the filter of Fig. 1 taken along the line 2-2.

The electromechanical wave filter shown in Fig. 1 comprises a mechanical portion 3 and electromechanical converters 4 at each end, rigidly mounted On a base 5. The mechanical portion 3 consists of a longitudinal bar 1 and three cross bars 8 which form a three-section filter of the type disclosed in my United States Patent 2,345,491 issued March 28, 1944.

Each electromechanical converter 4 comprises a piezoelectro crystal element 9, a metallic resonator II, and a support I2. The crystal 9 has one end attached to the end of the bar 1 and the other end to one side of the support l2. The resonator H has one end attached to the other length approximately equal length at the midband frequency of the filter. Each crystal 9 has a pair of electrodes l3 on its major faces. One pair of electrodes l3 are com nected, respectively, to the input terminals l5, l6 and the other pair to the output terminals [1, it. Since the ends of the crystal 9 and the resonator I! attached to the support l2 coincide with nodes or motion, the supports [2 may be secured at their lower ends to the base 5 by means of the screws I9 to provide a rigid mounting for the filter.

When an alternating electromotive force is impressed upon the input terminals I5, IS, the crystal 9 will vibrate longitudinally and set up corresponding longitudinal vibrations in the bar 7. However, there are likely to be set up also in the bar 1 undesired fiexural vibrations which interfere with the desired longitudinal vibrations and impair the proper operation of the filter. These extraneous flexural vibrations are more likely to occur in a rigidly mounted, multisection, filter of the cross bar type such as is shown in Fig. 1. In accordance with the invention, means are provided for damping the fiexural vibrations Without appreciably affecting the longitudinal vibrations, thereby improving the operation of the filter.

As shown in Figs. 1 means comprise a pair plates 2| attached to the and 2, these damping of piezoelectric crystal sides of the bar 1 which tend to flex. The plates 2| are preferably thin 45-degree Y-cut slabs from a Rochelle salt crystal or thin 45-degree Z-cut slabs from an ammonium dihydrogen phosphate crystal. A 45-degree Y-cut plate has its major faces substantially perpendicular to a Y axis and its length dimension inclinedat an angle of 45 degrees to the X and Z axes of the crystal. A 45-degree Z-cut plate has its major faces substantially perpendicular to a Z axis and its length dimension inclined at an angle of 45 degrees to the X and Y axes of the crystal. The plates 2| are preferably located at a point on the bar 1 intermediate two of the cross bars 8 and may be secured in position by means of melted Rochelle salt cement or urea formaldehyde cement, or they may be otherwise suitably attached. The plates 2| are provided with the usual electrodes 22 which are connected through an electrical impedance comprising a resistor R1. For maximum damping theresistance of the resistor R1 is ,made equal .to the sum .of the reactances of the two crystal plates '21 at the midband frequency of the filter.

The plates 2| are so poled that the voltages generated therein are aiding for the unwanted fiexural vibrations but opposing for the desired longitudinal vibrations. The energy associated withthe ilexural vibrations is thus dissipated in the resistorPtl and, therefore, this mode is efiectiyely dampedout, without appreciably affecting thepropagation of the longitudinal mode.

If theother sides ofthe bar 1 also tend to flex, asecondrpair of crystal plates 23 may be attached thereto .and provided with associated electrodes 24 and a second resistor R2. Furthermore, the damping action may bemade more complete by providingadditional damping means, such as the pair of crystal :plates 26, with electrodes 21 and resistor R3, and .the .plates .28, with electrodes 2% and resistor R4.

What is claimed is:

11. .Incombination, a mechanical vibratory element subjected to two modes of vibration and means for damping appreciably afiecting ,the other mode, said means comprising a pair of piezoelectric crystal plates associated withopposite sidesoi said element and "an electrical impedance connected between said plates, said plates being so poledthat the voltages generated therein are aiding for said one mode but opposing for said other mode.

2. The combination-in accordance with claim 1 1 in whichsaid one mode is nexural.

3. The combination inaccordance with claim l in which said other mode is longitudinal.

4. The combination in accordance with claim 1 in whichsaidonemode isflexural and said other mode is longitudinal.

5. The combinationinaccordancewith claim 1 in which saidimpedance comprises a resistor.

6. Thevcombinationin accordance with claim 1. .in' which said impedance comprises a resistor having-a resistance approximately equal to the sum of the reactan'ces of said crystal plates at an operating frequency of saidelement.

7. The -.combination in accordance with claim 1 in whicl i said element forms part of a mechanical wave .filter.

--8. Thecombination in accordance with claim 1 in'which said elementis in theform of .a bar.

9. The combination in accordance withclaim l in which said element constitutes a longitudinal barin a mechanical wave filter.

10. The combination in accordance with claim one of the modes without 1 in which said plates are thin, 45-degree Y-cut slabs from a Rochelle salt crystal.

11. The combination in accordance with claim 1 in which said plates are thin, 45-degree Z-cut slabs from an ammonium dihydrogen phosphate crystal.

12. Thecombination inaccordancewith claim 1 which includes additional damping means, similar to said first-mentioned damping means, :associated with said opposite sides of said element.

13. The combination in accordance with claim '1 which includes additional damping means, similar to said first-mentioned damping means, associated with the other sides of said element.

14. 'The combination in accordance with claim 1 which includes two additional damping means, each similar to said first-mentioned damping means, one being associated with said opposite sides or said element and the other being associated with the other sides of said element.

15. The combination in accordance with claim 1 whichincludes three additional-damping means, each similar to said first-mentioned damping means, one being associated with said opposite sides of said element and the other two .being associated with the other sides of said element.

16. In combination, a mechanical vibratory element, means for impressing longitudinal vibrations upononeend of said-element, and means for damping extraneous flexural vibrations in said element without appreciably affecting said longitudinal vibrations, said means comprising a pair of piezoelectric crystal plates attached to opposite sides of said element and an electrical impedance connected between said plates, said plates being so poled that .the voltages generated therein are aiding. for said viiexural vibrations-but opposing-,ior said longitudinal vibrations,

17. The combination in accordance with claim 16in which said impedance comprises .a resistor.

.18. The combination .in accordance with claim 16 in which said impedance comprises a resistor having .a .resistance approximately equal to .the .sum of the-reactances ofsaid .crystal plates at the frequency of said longitudinal vibrations.

'19. The combination in accordance with claim 16 in whichsaid element vforms part of a wave filter.

20. The combination in accordance withclaim 16 which .includes additional damping means, similar to said first-mentioned damping means, attached to said'opposite sides .of said element.

21. The-combination in accordance with claim l6 which includes additional damping means, similar to said first-mentioned damping .means, attached totheother sides of saidelement.

.22. ,In combination, tworigidly supportedielectrio-mechanical convertors, an interposed mechanical vibratoryelement along which longitudinal vibrations.aretransmitted from one. to the .other of saidconvertors, and means for damping extraneous fiexural vibrations in said element without appreciably affecting said longitudinal vibrations, said means comprising a pair of piezoelectric crystal plates attached to opposite .sides of said element and an electrical impedanee including a resistance connected between said plates, .said plates being so poled that the .voltages generated therein are aiding ,for said i'lexural vibrations but .opposingfor said longitudinal vibrations. I

23. The combination in accordance withclaim 22 in which saidv resistance has a value approxi- .:mately equal to the sum of the reactances 50f said crystal plates at the frequency of said longitudinal vibrations.

24. The combination in accordance with claim 22 which includes additional damping means, similar to said first-mentioned damping means, attached to said opposite sides of said element.

25. The combination in accordance with claim additional damping means, similar to said first-mentioned damping means, attached to the other sides of said element.

26. In combination, a longitudinal bar having 26 in which said resistance has a value approximately equal to the sum of the reactances of said crystal plates at the frequency of said longitudinal vibrations.

28. The combination in accordance with claim 26 which includes additional damping means, similar to said first-mentioned damping means, attached to said opposite sides of said longitudinal bar.

29. The combination in 26 which includes additional damping means, similar to WARREN P. MASON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,689,339 Harrison Oct. 30, 1928 1,788,519 Harrison Jan. 13, 1931 2 2,345,491 Mason Mar. 28, 1944 

